take advantage of the 'state hack' on the result of
(f y) :: State# -> (State#, Int) to expand the arity one more.
-There is a disadvantage though. Making the arity visible in the RHA
+There is a disadvantage though. Making the arity visible in the RHS
allows us to eta-reduce
f = \x -> f x
to
where
subst = mkCoreSubst env
old_rules = idSpecialisation in_id
- new_rules = CoreSubst.substSpec subst old_rules
+ new_rules = CoreSubst.substSpec subst out_id old_rules
final_id = out_id `setIdSpecialisation` new_rules
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